Carbon dots (CDs) and graphitic carbon nitride (g-C₃N₄) composites (CD/g-C₃N₄) were successfully synthesized by a hydrothermal method using urea and sugarcane juice as starting materials. The chemical composition, morphological structure and optical properties of the composites and CDs were characterized using various spectroscopic techniques as well as transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) results revealed new signals for carbonyl and carboxyl groups originating from the CDs in CD/g-C₃N₄ composites while X-ray diffraction (XRD) results showed distortion of the host matrix after incorporating CDs into g-C₃N₄. Both analyses signified the interaction between g-C₃N₄ and CDs. The photoluminescence (PL) analysis indicated that the presence of too many CDs will create trap states at the CD/g-C₃N₄ interface, decelerating the electron (e⁻) transport. However, the CD/g-C₃N₄(0.5) composite with the highest coverage of CDs still achieved the best bisphenol A (BPA) degradation rate at 3.87 times higher than that of g-C₃N₄. Hence, the charge separation efficiency should not be one of the main factors responsible for the enhancement of the photocatalytic activity of CD/g-C₃N₄. Instead, the light absorption capability was the dominant factor since the photoreactivity correlated well with the ultraviolet–visible diffuse reflectance spectra (UV–vis DRS) results. Although the CDs did not display upconversion photoluminescence (UCPL) properties, the π-conjugated CDs served as a photosensitizer (like organic dyes) to sensitize g-C₃N₄ and injected electrons to the conduction band (CB) of g-C₃N₄, resulting in the extended absorption spectrum from the visible to the near-infrared (NIR) region. This extended spectral absorption allows for the generation of more electrons for the enhancement of BPA degradation. It was determined that the reactive radical species responsible for the photocatalytic activity were the superoxide anion radical (O₂^•−) and holes (h⁺) after performing multiple scavenging tests.

Carbon dots (CDs) were successfully derived from water hyacinth leaves and the binary composite was achieved by incorporating CDs with g-C₃N₄ through hydrothermal treatment. The average particle size of CDs was found to be 3.1 nm and a blue-green fluorescence was emitted under the UV light irradiation. Both of the composites loaded with 20 wt.% (20CDs/g-C₃N₄) and 40 wt.% (40CDs/g-C₃N₄) of CDs achieved the highest degradation efficiency of 2,4-dichlorophenol (2,4-DCP) with 1.7 times higher than that of pure g-C₃N₄. This work successfully improved the properties of g-C₃N₄ by elongating the lifetime of photogenerated electrons and widening the visible light response. Both of 20CDs/g-C₃N₄ and 40CDs/g-C₃N₄ recorded the highest photocatalytic performance in degrading 2,4-DCP with degradation rate constant of 0.0194, and 0.0186 min⁻¹, respectively. This is contributed by the prolonged charge carrier lifetime in 20CDs/g-C₃N₄; good visible light absorption and high specific surface area in 40CDs/g-C₃N₄. For the scavenger test, hole (h⁺) and superoxide radical (^·O₂⁻) were acknowledged as the key active species in photocatalysis.

This study produced a novel polysulfone (PSF) membrane for dye removal using lemon-derived carbon quantum dots-grafted silver nanoparticles (Ag/CQDs) as membrane nanofiller. The preparation of CQDs was completed by undergoing hydrothermal treatment to carbonize the pulp-free lemon juice into CQD solution. The CQD solution was then coupled with Ag nanoparticles to form Ag/CQDs nanohybrid. The synthesized powders were characterized in terms of morphologies, functional groups and surface charges. A set of membranes was fabricated with different loadings of Ag/CQDs powder using the nonsolvent-induced phase separation (NIPS) method. The modified membranes were studied in terms of morphology, elemental composition, hydrophilicity and pore size. In addition, pure water flux, rejection test and fouling analysis of the membranes were evaluated using tartrazine dye. From the results, 0.5 wt % of Ag/CQD was identified as the optimum loading to be incorporated with the pristine PSF membrane. The modified membrane exhibited an excellent pure water permeability and dye rejection with improvements of 169% and 92%, respectively. In addition, the composite membrane also experienced lower flux decline, higher reversible fouling and lower irreversible fouling. This study has proven that the addition of CQD additives into membrane greatly improves the polymeric membrane’s properties and filtration performance.